JPS63164484A - Semiconductor laser element - Google Patents

Abstract

PURPOSE:To suppress the lateral expansion of a current by providing an optical guide layer between an active layer as an active region and a clad layer, raising the resistance value of the guide layer on the side of a current being constricted and reducing the resistance value of the guide layer on the opposite side. CONSTITUTION:A buffer layer 12, a clad layer 13, an n-type GRIN layer 14 which operates as an optical guide layer, an active layer 15, an undoped GRIN layer 16, a clad layer 17 and a cap layer 18 are continuously grown by a molecular beam epitaxial growing method on an n-type GaAs substrate 11. A route in which a current expansion occurs except a mesa is mainly of the GRIN layer 16 having p-10<15>cm<-3> of relatively high resistance, and under the layer 15, the low resistance layer 14 and the n-type AlGaAs layer 13. Thus, the current expansion until the current is injected from the mesa to the active layer can be suppressed to extremely small value.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to the structure of a semiconductor laser device that oscillates with an extremely low threshold current.

<Prior art> Conventional semiconductor lasers are classified into gain waveguide type and refractive index waveguide type based on the optical waveguide mechanism, but from the viewpoint of transverse mode stability, which is important in practical terms, the refractive index waveguide type Semiconductor lasers with various waveguide structures have been developed, which are more advantageous than gain waveguide types. In general, index-guided semiconductor lasers confine light and current in a narrow excitation region, which is effective in lowering the threshold current required for oscillation. B is a typical laser device structure with such a low threshold current.
H (Buried Heterostructure
) type semiconductor lasers and ridge waveguide type semiconductor lasers are well known.

The BH type semiconductor laser shown in FIG. 2 is formed by depositing a double heterojunction structure in which a laser oscillation active layer 3 is sandwiched between a pair of cladding layers 2.4 from both sides on a substrate 1, and then forming it into a mesa shape. Both sides of the mesa structure are filled with a low refractive index material 14, and by narrowing the waveguide width W to about 2 fim, fundamental transverse mode oscillation can be made to have a low threshold current of about 10 mA or less. In order to effectively constrict the current in the mesa-type region used for oscillation, the buried layer I4 should be a high-resistance layer or have a multilayer structure having a pn junction that provides reverse bias.

However, since the buried layer 14 is grown after the mesa region is formed, an interface level is formed at the interface between the buried layer 14 and the mesa region. Therefore, the reactive current that flows through this interface without passing through the active region is 2 to 10 mA.
It exists to some extent. The ratio of this reactive current to the total current is
As the waveguide width W becomes narrower, it becomes larger, so there is a limit to lowering the threshold value.

On the other hand, in the ridge waveguide laser shown in FIG. 3, part of the p-type cladding layer 4 above the active layer 3 of the double heterojunction structure is formed into a mesa shape, and the isometric refractive index is It was designed so that it would be low. Ridge waveguide type is B
Unlike the H type, the active layer 3 remains on the entire surface, so that most of the current flows through the active layer 3. However, since the p-type cladding layer 4 is left in areas other than the mesa part, the current spreads outside the mesa part in the p-type cladding layer 4 as shown by the arrow in FIG. The current flows away from the current and becomes a reactive current that is not used for oscillation. This reactive current results in an increase in threshold current.

<Summary of the Invention> In view of the above-mentioned problems, the present invention aims to reduce reactive current that spreads in the lateral direction and does not contribute to oscillation in a semiconductor laser having a structure of a ridge guide type or a similar current path. Accordingly, it is an object of the present invention to provide a semiconductor laser device with a low threshold current.

To achieve this objective, the semiconductor laser of the present invention includes:
5separate configurationH with a light guide layer between the active layer and the cladding layer as an active region
The lateral inward spread of the current is suppressed by adopting a SCH (SCH) structure and increasing the resistance value of the light guide layer on the side where the current is constricted and lowering the resistance value of the light guide layer on the opposite side. It is characterized by

<Example> FIG. 1 is a schematic cross-sectional view of a semiconductor laser device showing one example of the present invention. n-GaAs substrate 11 (S i =
n-GaAs buffer layer 12 (S i = 2 × 1018 cm”+
0.5 pm thickness) + n-A7'0.7 GaO,R
As cladding layer 1 B (S i ~2 x 1018
cm+-8+ 1-4pm thick).

Works as a light guide layer (n-AlxGa1-xA5aGR
IN (Graded-Index) layer 14 (S
i=5×1017m-8゜0.2μm thick), undoped GaAs quantum well active layer 15 (60 people), undoped A
l * G a l −X A 8 @ G R I N
Layer 16 (p ~ 1015 < 8.0.2.4 m thick),
I)-AI! 0.7GaO, 8As cladding layer 17 (
Be"5X 110l7+-3,1,4 μm thick).

p-GaAs cap layer 18 (B e = 5
10'8csr8° 0.5 μm thick) is continuously grown by MBE (Molecular Beam Epitaxial Growth) method. GRI
The N layers 14 and 16 are from the active layer 15 to the cladding layer IR, 17
The Al mixed crystal ratio X changes sequentially from 0.2 to 0.7 to exhibit a square distribution, and has a light guide function. After growth, RIBE (Reactive
Etching is performed up to just before the GRIN layer 16 using a reactive ion beam etching method to form a striped mesa. The p-type cladding layer 17 is etched to a depth of 500^ or less in areas other than the stripe portion. Next, a SiNx film 19 is formed by plasma CvD method except on the cap layer 18, and p
The side electrode 20 is made of AuZn/Au, and the n-side electrode 21 is made of AuGe/Ni/Au.

The stripe width under the mesa is 3.5μm, and the cavity length is 25μm.
The semiconductor laser device with a diameter of 0 μm oscillated at an oscillation wavelength of 840 nm, and the threshold current was 4 mA.

In the semiconductor laser device described above, the path that causes current spread other than the mesa portion is mainly the p, 1015, -8: GRIN layer 16 of relatively high resistance, and the area under the active layer 15 is the n-
GRIN layer 14 and n-A77GaAs cladding layer 13
Therefore, the current spread from the mesa portion to the time when the current is injected into the active layer is suppressed to be extremely small. In this way, it is possible to realize a low threshold current by effectively utilizing the low threshold 'current density characteristics of the GRIN-5CH structure quantum well semiconductor laser device.

FIG. 4 is a schematic cross-sectional view of a semiconductor laser showing another embodiment of the present invention. A multilayer structure with an active layer defined by a double heterojunction similar to that shown in Fig.
After fc growth on the aAs substrate 11, a striped ridge is formed using the Si02 film as a mask, and an n-GaAS current blocking layer 30 (Si = 2
X 10'8 ct-8 + 2 μm thick) is selectively grown outside the ridge. After removing the 5i02 film.

AuGe/Ni/Au is formed as the p-side electrode 20 and AuZn/Aun-side electrode 21.

The stripe width under the mesa is 4μm, and the cavity length is 250μm.
In the case of a semiconductor laser element with m, it oscillates at 840 nm,
The threshold current was 8 mA. In this laser device as well, since the p-type rad layer 17 is thinned to 500 Å or less in areas other than the mesa portion, the lateral inward spread of the current is suppressed to an extremely small value for the same reason as in the embodiment shown in FIG. ing.

In this example, since the transverse mode is stabilized using light absorption by the n-GaAs layer 30, fundamental transverse mode oscillation can be obtained up to a relatively wide stripe width of about 5 μrrLl. However, since the absorption loss is large, the first The threshold current is higher than in the structure shown in the figure.

In the above two embodiments, the light guide layer 14 on the n-type cladding layer side
S i = 5 x 1017>-3+ The optical guide layer 16 on the p-type cladding layer side is undoped with a thickness of 9 to 10166 m.
-1, but in order to suppress the spreading current, the carrier concentration of the n-side optical guide layer should be n≧1017Ll so that the n-side optical guide layer has a low resistance and the p-side optical guide layer has a high resistance.
! The carrier concentration of the optical guide layer on the p side may be n-type I+1-3 and p-type where p≦1.016 cll-8. Furthermore, the dopant in each layer can be S I+ Te+ se or the like for n-type, and B er Z n+ Mg or the like for p-type.

Furthermore, in high-purity MBE growth as in the above example, it is possible to obtain a p-type of 1014"etrB or less when undoped. In other growth methods such as LPE (liquid phase epitaxial growth), it is also possible to obtain the above-mentioned range when undoped. If n-type or p-type is obtained, the optical guide layer may be undoped as appropriate.Furthermore, the p-side optical guide layer has n≦1017.
It may be an n-type of "r".

In this case, since the holes injected into the n-type optical guide layer are minority carriers, the diffusion length is limited to a short diffusion length of 1 μm or less, and the spreading current is suppressed. Also, 1017 , ;8
At carrier concentrations below, recombination in the guide layer is smaller than radiative recombination in the active layer, and loss of injected carriers as a remote junction does not become large.

The device structure may be any structure as long as the p-side rad layer is thinned to 500λ or less or completely removed outside the oscillation region and current is blocked, as seen in the above embodiments. For example, the current blocking layer 30 in the embodiment of FIG. 4 may be semi-insulating GBAs or AlGaAs or even a resin such as polyimide.

In the above embodiment, the active layer has a quantum well structure with a width of 60λ in order to obtain a low threshold value, but the active layer generally has a quantum well structure with a width of several hundred λ to 2000λ.
A layer with a thickness of about λ can be formed using the usual LPE method or MO-CVD.
It may be formed by a method. Further, the optical guide layer may not be a GRIN layer but may be a layer having a constant composition or a GRIN layer having a composition change other than a square distribution. Furthermore, the material is AlGaAs
Not limited to InGaAsJ? system j InAIGaP)
System, AI! GaAsSb system.

It can be widely applied to semiconductor lasers using InGaA/As-based materials and the like.

<Effects of the Invention> As explained in detail above, according to the present invention, a low threshold semiconductor laser can be realized without etching the active layer. There is no decrease in reliability and it is extremely useful in practice.

[Brief explanation of the drawing]

FIG. 1 is a schematic cross-sectional view of a semiconductor laser device showing one embodiment of the present invention. FIG. 2 is a schematic cross-sectional view of a conventional BH laser. FIG. 3 is a schematic cross-sectional view of a conventional Norifuji waveguide type laser. FIG. 4 is a schematic cross-sectional view of a semiconductor laser showing another embodiment of the present invention. 11...n-GaAs substrate 12- n-GaAs buffer yN 13-n-clad i 14...n-
GRIN layer 15...Quantum well active layer 16...Undoped GRIN layer 17...p-cladding layer 18
...cap layer

Claims (1)

[Claims] 1. A semiconductor laser device in which a stripe structure for confining the injection current is added to a double heterojunction structure consisting of an active layer and a pair of cladding layers, and an optical guide layer is inserted between the active layer and each of the cladding layers. 2. A semiconductor laser device according to claim 1, wherein the light guide layer on the side closer to the stripe structure is set to a high resistance value, and the light guide layer on the opposite side is set to a low resistance value.